Even if what she said is technically true, under a strict definition, a licence agreement, or some obscure court ruling, it sounds like your Academic ethics speaker has her priorities seriously out of whack.
Plagiarism as a concept doesn't really rest on intellectual property concepts. I'm sure that, if your school is run by real scum, or you invent something valuable enough to fight over, they will claim that they own your work; but that isn't what makes plagiarism a problem.
Plagiarism is any unattributed inclusion of outside material into a given piece of your work(unless such outside material is considered to be common/trivial knowledge within your discipline. It has nothing whatsoever to do with who "owns" the material. So, you can cite yourself, you can cite the work of others(whether or not it is their property, and whether or not they agree to the citation), you can cite public domain works and so on. You just cannot include an outside element without citation.
She should have just discussed how plagiarism is a form of academic fraud, misrepresentation of the source of material, and left the whole stupid discussion of property out of it. Her argument makes it sound like you aren't allowed to use other people's writings at all, while it would be ok to copy wholesale from anybody's work as long as you bought their property rights first. That is incredibly stupid. I can't believe they actually let people like her lecture to university students.